THE EDGE of the solar system!

Both Voyager 1 and Voyager 2 continue to send data to Earth more than 30 years after they first launched. During the 1990s, Voyager 1 became the farthest manmade object in space.

Each spacecraft has now crossed the edge of the solar system, known as termination shock, where the outbound solar wind collides with inbound energetic particles from interstellar space. The termination shock surrounds the solar system and encloses a bubble called the heliosphere.

"The solar wind is blowing outward trying to inflate this bubble, and the pressure from interstellar wind is coming in," said Edward Stone, physicist and Voyager project scientist at Caltech in Pasadena, Calif. He and other researchers published a series of studies in the journal Nature this week that detail the Voyager findings.

Voyager 2 reached the southern edge of the solar system 7.8 billion miles (84 AU) from the sun, closer than Voyager 1 which had reached the northern edge 8.7 billion miles (94 AU) from the sun. That confirms earlier suspicions about the heliosphere bubble being squashed at its southern region.

The reason for that asymmetrical shape rests with an interstellar magnetic field that puts more pressure on the southern region of the solar system — something that may change over 100,000 years as that magnetic field experiences turbulence, Stone said.

"We're actually seeing the shock for the first time," said John Richardson, principal scientist for Voyager's Plasma Physics instrument at MIT in Cambridge, Mass.

An added mystery remains as to why the solar wind slows down early, as though anticipating running headlong into the termination shock. Researchers have begun looking into whether the solar wind somehow sheds energy ahead of time.

"Somehow the solar wind knows the shock is coming before it gets there, and theory says that shouldn't be," Richardson noted, adding that the solar wind speed drops from its supersonic speed of about 248 miles per second (400 km/s) to 186 miles per second (300 km/s) even before hitting the edge of the solar system. That speed falls more noticeably to about 93 miles per second (150 km/s) after the termination shock.